Fix passing arguments to VM dylib on Apple M1

The engine function pointer to vmMain used variadic arguments but
the vmMain function in the dylib has explicit arguments. Evidently
the arguments are stored on the stack and/or registers differently.

Found by fretn.

code/sys/sys_main.c

@@ -579,7 +579,7 @@ Used to load a development dll instead of a virtual machine
 =================
 */
 void *Sys_LoadGameDll(const char *name,
-	intptr_t (QDECL **entryPoint)(int, ...),
+	vmMainProc *entryPoint,
 	intptr_t (*systemcalls)(intptr_t, ...))
 {
 	void *libHandle;